The present invention is generally directed to improvements in color television receivers. It is particularly directed to receiver processing systems which enhance the bandwidth of detected color-difference signals in response to certain luminance transitions.
Broadcast television signals are conventionally derived from wide band R (red), B (blue) and G (green) primary signals. The primary signals are combined so as to develop a wideband Y (luminance) signal. In addition, the primary signals are stripped of some of their high frequency components to develop narrower bandwidth chrominance or color-difference signals, R-Y, B-Y and G-Y. The latter signals are normally transmitted as quadrature amplitude modulated I and Q signals along with the Y signal. Generally, the I signal has a greater bandwidth than the Q signal.
At the receiver, the Y signal is detected and applied to a CRT (cathode ray tube) to develop a wide bandwidth black and white image. The color components of the image are usually developed by treating the I and Q signals equally. This is, both are detected and processed as though they had equally narrow bandwidths, and are applied to the CRT so as to combine with the Y signal thereat.
Although the above-described signal processing technique generates commercially acceptable images, it is known that visible image errors are developed. For example, combining each color-difference signal with equal proportions of the Y signal at the receiver effectively attributes to each color-difference signal an equal portion of the high frequency Y components. In the general case, the primary colors do not contribute equally in the development of the high frequency Y components as the television signal is normally developed. Hence, attributing to them equal high frequency Y components at the receiver produces noticeable errors in the reproduced television image.
To overcome the problem described above, it has been proposed that, under certain conditions, each of the three color-difference signals detected in the receiver be modified so as to include its own unique, controllable portion and polarity of the high frequency components of the Y signal. U.S. Pat. No. 4,181,917 discloses that this may be accomplished by inferring that the high frequency components of the Y signal should be included in each color-difference signal according to the ratio which the derivative of each color-difference signal's low frequency components bears to the derivative of the Y signal's low frequency components. Thus, each color-difference signal is combined with a different amount and the appropriate polarity of "inferred highs" contained in the Y signal. This process is referred to herein as "chrominance bandwidth enhancement."
It has been proposed that such chrominance bandwidth enhancement be effected only under certain circumstances, such as when the derivative of the Y signal exceeds a predetermined amplitude. When the Y signal reflects a luminance step or impulse, the derivative of the Y signal is usually large, wherefore a decision is made to implement bandwidth enhancement. A problem with such a decision process is that it assumes that the enhancement scheme is able to accurately determine the ratio which the derivative of the low frequency color-difference components bears to the derivative of the low frequency Y components. Since both derivatives approach zero near the edges of a luminance transition, it is very difficult to obtain an accurate ratio determination at those times.
A particularly troublesome transition is an isolated impulse. Because the derivative of such a transition goes to zero near the center of the transition, and because the amplitude of that derivative is used to decide whether to activate the bandwidth enhancement scheme, the enhancement may be turned on, turned off when the derivative goes to zero, and then turned on again as the amplitude of the derivative increases. Consequently, the decision as to when to activate the enhancement scheme is subject to error.
For the reasons stated above, prior chrominance bandwidth enhancement systems have introduced certain errors in the television image while attempting to correct other errors. Consequently, commercial television receivers have not adopted such enhancement schemes. The present invention substantially eliminates the errors in the decision as to when to activate chrominance bandwidth enhancement, thereby rendering such a scheme more commercially viable.